Aircraft, including helicopters, are generally fitted with one or a plurality of turboshaft engines, the principle of which is to drive a turbine in rotation through the combustion of a gas into which fuel is injected.
At the outlet from the turbine, the burnt gas that has driven the turbine in rotation, known as exhaust gas, is discharged to the exterior through an exhaust pipe. The cycles of the turboshaft engine result in exhaust gas temperatures of approximately 600° C. The theoretical thermal energy contained in this flow of exhaust gases is estimated to be 60% of the potential energy contained in the fuel injected at the turbine inlet.
It is therefore advantageous to attempt to recover part of this thermal energy in order to increase the efficiency of the turboshaft engine. To do this, solutions have been proposed, in particular the use of heat exchangers situated in the exhaust pipe of the turboshaft engine, allowing part of the thermal energy to be recovered. This recovered thermal energy is used, for example, to preheat the gas supplied to the turboshaft engine before it is burned, or to reheat a gas from a secondary machine present in the aircraft, of a turbine engine or piston machine type.
However, these solutions lead to a multitude of disadvantages. This is because the presence of a heat exchanger in the exhaust pipe results in pressure losses that affect the operation of the turbine. This heat exchanger can lead to fouling that affects the performance of the turboshaft engine, and requires appropriate washing procedures, and also to degradation in the event of blade loss and blade-shedding. Blade-shedding is a mechanical protection against overspeed of the free turbine of the turboshaft engine.
In addition, the use of such a heat exchanger requires the turboshaft engine to be calibrated for operation with this heat exchanger. In the context of use of the heat exchanger to preheat the gas fuelling the gas turbine, the presence of the heat exchanger requires an engine operating point that is different from the operating point without a heat exchanger, which means that the engine performance is heavily affected if the heat exchanger is not used. This non-use of the heat exchanger, if it is accidental (due to a failure of the heat exchanger), can furthermore cause degradation of the heat exchanger and non-operation of the engine.
Finally, the constraints on the operation of the heat exchanger in an exhaust pipe (high temperatures greater than, or equal to, 600° C., pressure between 4 and 8 bar, etc.), require an appropriate sizing of the heat exchanger resulting, in particular, in an increase in its size and weight, and the use of materials that can withstand these constraints. However, the thermal conduction performance of these materials that are adapted to withstand the constraints is generally poor, which reduces the efficiency and usefulness of the heat exchanger.